The present invention relates to a motor control device for controlling the drive of a motor when it is overloaded.
An example of a conventional control device is shown in the U.S. Pat. No. 4,473,856. This control device detects a load current of a DC motor, and stops the supply of power to the DC motor when the detected load current is more than a predetermined value. If the control device stops the supply of power to the motor, then the control device continues to stop the supply of power to the motor, as long as power is applied to the control device. The control device clears the shutdown of the motor by manual operations to stop the supply of power and reapply power to the control device.
In the case of a drive motor of an air-control door such as an air-mix door and a blower in an airconditioning system for an automobile, for example, the drive motor is locked or overloaded when any foreign substance is caught in the drive mechanism of the air-control door or blower fan, interrupting the rotational movement of the air-control door or the rotation of the fan. In such cases, the release of a foreign substance can return the drive motor to a normal operation when the supply of power to the drive motor is stopped resulting in a sudden removal of the torque of the drive motor from the foreign substance. A foreign substance can also be released by rotating the drive motor in reversely to apply force reverse to it, and that as a result, the operation of the air-control door or the blower becomes normal. Further, in cases when the drive motor is overloaded because of unsmooth operation of the drive mechanism for driving the air-control door, the drive mechanism can be enabled to operate smoothly by repeating the regular and reverse driving of the drive motor.
According to the prior control device, however, the state of shutdown of the motor is continued as is, even when the locked or overloaded condition is cleared after the foreign substance is removed, by stopping the supply of power to the motor. Namely, driving of the motor is not resumed automatically. Also, according to the prior control device, the motor is left at the shutdown state when it is locked or overloaded, therefore, it is not possible to obtain the effect of repetitive regular/reverse driving of the motor. Further, in the prior control device, in order to know whether the motor is released from the lock or overload state, a problem of manually reapplying power to the control device after stopping the supply of power to the motor must be done repeatedly.